Incredible quadruple transit on Saturn!

By Phil Plait | March 17, 2009 10:45 am

The folks at Hubble just released this fantastic image!

Hubble sees a quadruple transit of moons across Saturn

OK, duh, that’s Saturn. But you can see four moons crossing its face at the same time! Such an event is pretty rare, and very cool. The moons are (from left to right) Enceladus, Dione, Titan, and Mimas. You can also see the shadows of Enceladus and Dione on the planet’s cloudtops, too.


Saturn orbit diagram showing how rings appear

Saturn, like the Earth, is tilted with respect to its orbit around the Sun. Our tilt is about 24 degrees, and Saturn’s is about 27. This means that twice every Saturn year (which are roughly 30 Earth years in length) we see Saturn’s rings edge-on. They can get so thin they practically disappear! That happens in September of this year, and as you can see from the image above image, our viewing angle of the rings is currently very shallow.

The icy particles in the rings orbits over Saturn’s equator, just as the moons do. That means that if we’re seeing the rings nearly edge-on, the orbits of the moons are that way as well. This makes transits — moons moving across the face of the planet — more common. So on February 24 of this year, Hubble was able to snap a spectacular series of images of four of Saturn’s moons projected on Saturn’s visage. You can see how the moons moved in the image below, showing the time-sequence Hubble took of the event.

Sequence of Saturn transit images

Besides the relative rarity of this event, what strikes me the most is how seriously frakkin’ big Titan is: if Titan were orbiting the Sun, there’s be no question is would be a planet; it’s easily bigger than Mercury! This also shows just how monstrous Saturn is; over 800 Earths could fit inside it. Saturn’s gravity is so powerful in can hold all those moons in sway, keeping them in their clockwork orbits for billions of years.

I’ll add that I went out to look at Saturn a few days ago; it’s up when the Sun sets, easily visible in the constellation of Leo. I couldn’t help but notice how faint it looked compared to last year, and I realized with a start that this was because the rings appear so thin right now. They reflect so much light from the Sun that when we see them edge-on, Saturn is visibly dimmer! It really brought home to me that Saturn is not just a wandering point of light in the sky, but a world in and of itself, and in fact a system of worlds which interact with each other. And they dance around each other, as they have danced for eons, and will for eons to come.

Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (42)

  1. Helioprogenus

    It’s specifically pictures like these that make me angry at NASA for delaying the Hubble Repair Mission. Do they not know that these amazing photos generate interest and actually add to our understanding of the universe in ways that the ISS cannot? Please NASA, get your head out of your err…flan and do something about it. I know the money and resources committed to the ISS are a priority, but sometimes, the priority lies in what garners fascination, insight and a great deal of interest. Hubble keeps numerous astronomers, post-docs, and even amateur sky gazers employed.

  2. Bigfoot

    We are at 24 degrees tilt? It’s not 23 degrees? Was it closer to 23 degrees before, back when globes were designed and the tropic lines, etc. were mapped out? I know we precessed around the axis on a many-thousand year scale, but I didn’t know out tilt changed!

    Inquiring minds want to know!

  3. what strikes me the most is how seriously frakkin’ big Titan is

    That’s the first thing that struck me when I saw the picture. Wow!

    if Titan were orbiting the Sun, there’s be no question is would be a planet

    But Phil, don’t you know that Titan does orbit the Sun? 😉 Seriously though, this brings up one of my objections to one of your objections to the current defintion of a planet. You wrote a while back that if Earth’s orbit were as far out as Pluto’s then it wouldn’t clear its orbit and so wouldn’t be considered a planet. You went on to suggest that it was ridiculous that a body be considered a planet or not just based on where its orbit lies. I realize that the analogy between that and Titan’s situation isn’t perfect, but I would certainly say that the nature of a body’s orbit is (for me) an important factor in determining whether or not to call it a planet.

    Thanks for the beautiful picture!

  4. DrFlimmer


    The delay of the SM4 was a good choice. Hubble had a failure that will be fixed in the upcoming mission. We were lucky that the failiure occured before the launch, otherwise we could have lost Hubble forever!

    Btw: nice picture, indeed!

  5. Now, the amateur images of such transits, esp. of Titan and its shadow, aren’t that much worse – have a look at this collection of pics from Oz and AZ of the March 12 event, for example.


    The sight of objects beyond Earth never gets old, does it?


    Dr. Phil Plait:

    Besides the relative rarity of this event, what strikes me the most is how seriously frakkin’ big Titan is: if Titan were orbiting the Sun, there’s be no question is would be a planet…

    What? Err… Phil, I think that you had meant to say: if Titan were orbiting the Sun, there would be no question of it being a planet… 😉

  8. Mount

    That’s cool! Knowing that the moons are on the same plane as the rings helps to give a bit of depth perception to the picture, makes it feel more 3 dimensional. You can easily tell which moons are closer to Saturn. I also like seeing Enceladus getting ready to overtake Dione in the sequence image.

    I looked and didn’t see anywhere, how much time elapsed between the first and last shots in that sequence?

  9. Is Saturn really that far out of round? I know planets aren’t true spheres, but wow!

  10. Yes, Saturn is noticeably oblate – it rotates over twice as fast as Earth (10.2 hours) and is made of almost all fluid, so it bulges around the middle.

    Earth is also slightly oblate too, but only by about 1% difference from a perfect circle.

  11. Question. Why does Titan show up where it does. If we are seeing the rings edge on shouldn’t all of the moons be in that plane, or is that just a perspective thing?

  12. Trebuchet

    Tony, that was my question as well. I think the answer is that it orbits on a much larger radius than the outside of the rings so the tilt, small as it is, brings it much higher up when everything is projected into 2D. If the rings were truly edge-on it would overlap them. Hopefully someone can confirm.

  13. DrFlimmer

    @ tony tony tony:

    The moon isn’t surrounding the earth always in the same plane. That’s the reason why we have relatively few solar and moon eclipses.

  14. Too cool. What a pity the picture is fake. Everyone knows Saturn is flat.

    C’mon Phil, how much is NASA paying you?


  15. Titan’s orbit is tilted about 0.1 degrees with respect to the equatorial plane of Saturn. That, however, would only get you about 3% of a Saturn radius vertically out of the plane though. The bigger effect is that Earth is still well out of the ring plane and, as someone noted, a projection effect makes Titan look a lot higher up on Saturn than the edge of the rings. (Titan is around 10 times farther from the planet, so it’ll always look higher.)

  16. Trebuchet

    John Weiss, thanks, that’s what I was thinking. It occurred to me that assuming Titan to be in the same plane as the rings, you could actually take some measurements on the picture to estimate how much farther out it is. I’m probably too lazy to actually do it, of course!

  17. Trebuchet

    And I keep forgetting to say what a truly awesome picture that is.

  18. Jeremy

    So which image is correct … the NASA time series of Titan’s transit (CCW) or the screensaver advert on the right side of the blog (CW)?

  19. Thanks for all the answers.
    I have an Aero Eng degree but have done nothing but aircraft. I do enjoy space (spacecraft dynamics was a brief focus) but it blows my mind how people like BA and others can look at this stuff and interpret it for what it is (Example, the picture of Triton the other day).
    I think I’m gonna go transit a Guinness.

  20. amphiox

    I’ve always believed that when it comes to definitions, for the purposes of general communication, we should be lumpers, and for the purposes of specific researchs, we should be splitters.

    We really should be classifying planets with subcategories, at least one for the Plutos and Ceres type objects, and at least one of which should be used for all the large moons. The smaller star in a binary is still considered a star, after all.

  21. Torbjörn Larsson, OM

    It’s a (teeny, weeny bit of a) bummer that the next flagship mission revisit Jupiter instead of Saturn, since Titan seems to be the closest atmospheric and surface analog to Earth nearby.

  22. Deepak

    Why there is no shadow of the rings on Saturn?

  23. MPG

    Why there is no shadow of the rings on Saturn?

    There is. Look at the thin dark band above the rings as they pass in front of Saturn. Compare with the offset of the shadows from Enceladus and Dione to give you an idea of what direction the light is coming from.

  24. darth_borehd

    What are the two white dots on either end of Saturn? Are those other moons?

  25. IVAN3MAN

    @ darth_borehd,

    Dr. Phil Plait provided the answer to your questions when he stated: “The moons are (from left to right) Enceladus, Dione, Titan, and Mimas. You can also see the shadows of Enceladus and Dione on the planet’s cloudtops, too.”

  26. S Johnson

    Hey, would you mind never using the fake word “frak” or “frakkin'” again? This is actually real life, not an episode of Battlestar Galactica.

  27. You always seem to be one step ahead of the curve on things like this.

  28. Sean Crowell

    I’m actually quite curious: How does a respectable magazine allow something like this to make it into their website content: “. . . seriously frakkin’ big Titan is: if Titan were orbiting the Sun, there’s be no question is would be a planet.” Stuff like this really does affect your credibility. If I wanted to read amateur writing I could find many other sources.


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